In the production of chemical pulp of high brightness, wood chips are first cooked to separate the cellulose fibers. During the cooking, part of the lignin holding the fibers together is degraded and modified such that it can be removed by subsequent washing. However, in order to achieve sufficient brightness, more lignin must be removed, together with brightness impairing (chromophoric) groups. This is frequently effected by delignification with oxygen, followed by bleaching in several stages.
A conventional bleaching sequence for a digested lignocellulose-containing pulp, e.g. kraft pulp from softwood, is (C+D) E.sub.1 D E.sub.2 D, where (C+D)=chlorine/chlorine dioxide stage, E =alkaline extraction stage, D=chlorine dioxide stage. The (C+D) and E.sub.1 stages are defined as prebleaching stages. The sequence D E.sub.2 D is called final bleaching.
If an alkaline oxygen stage is used before the prebleaching sequence of multi-stage bleaching of, for example, kraft pulp, it is possible to reduce the discharge by more than half the original amount, since spent oxygen bleach liquor not containing chlorine is recoverable. However, after an oxygen delignification stage, the lignin remaining in the pulp is about half of the amount remaining after the digestion in the cooking process, which thus at least partly has to be dissolved out of the pulp. This is achieved in the subsequent bleaching.
Bleaching of chemical pulps is mainly carried out with chlorous bleaching agents, such as chlorine, chlorine dioxide and hypochlorite, resulting in spent bleach liquors containing halogenated organic compounds and chlorides. The corrosive tendency of the latter, makes it difficult to close the bleach plant and the halogenated organic compounds mean discharges detrimental to the environment. Therefore, nowadays there is a strive towards the use of, to the greatest possible extent, bleaching agents poor in or free from chlorine, so as to reduce the discharges and make possible the recovery of spent liquors. Examples of such bleaching agents are peroxides, e.g. inorganic peroxides, such as hydrogen peroxide and sodium peroxide, and organic peroxides, such as peracetic acid. The formation of compounds detrimental to the environment is especially pronounced in the prebleaching, where the content of lignin is high. Therefore, the greatest effect of a change to bleaching agents which are less harmful to the environment, such as hydrogen peroxide, is obtained in the prebleaching. In actual practice, however, hydrogen peroxide is not used to any appreciable extent in the first stage of a bleaching sequence to obtain an initial reduction of lignin and/or an increase in brightness, because of the large amounts of added hydrogen peroxide which are necessary.
Thus, large amounts of hydrogen peroxide must be added in alkaline hydrogen peroxide treatment to obtain a satisfactory dissolution of lignin, since such a treatment gives a high degree of decomposition of the hydrogen peroxide, resulting in considerable costs for the chemicals. In acidic hydrogen peroxide treatment, the same dissolution of lignin can be obtained as in alkaline treatment with a much lower consumption of hydrogen peroxide. However, the acidic treatment results in a substantial drop in the viscosity of the pulp, i.e. the decomposition products of the hydrogen peroxide, at low pH values attack not only the lignin, but also the cellulose, so that the length of the carbohydrate chains is reduced, resulting in impaired strength properties of the pulp.
According to SE-A 420,430, this drop in the viscosity in an acidic hydrogen peroxide treatment can be avoided by carrying it out in the presence of a complexing agent, such as DTPA (diethylenetriaminepentaacetic acid), at a pH of 0.5 to 3.0. This treatment step is followed by an alkaline extraction stage for removal of dissolved lignin, without intermediate washing.